PTEN (phosphatase and tensin homologue deleted on chromosome ten) is a tumor suppressor gene having both protein and lipid phosphatase activity. By its lipid phosphatase activity, it catalyzes dephosphorylation of D3 position of the myo-inositol ring of phosphatidylinositides, and thus counteracts the activity of phosphatidylinositol 3-kinase (PI3K). Our goal is to understand the mechanism underlying the anti-proliferative effect of PTEN. Our preliminary data indicates that PTEN co-localizes with nucleophosmin (B23)in the nucleolus of T cells. B23 is thought to be involved in ribosomal RNA processing. We want to investigate whether PTEN interacts with B23, and if so, whether this interaction has any consequence in B23-mediated ribosomal RNA processing, which may give us some insight in the antiproliferative effect of PTEN. We are also interested in mapping out the domain of PTEN responsible for the interaction with B23.